X-ray absorption has been used as the basis for screening objects both to create some form of representational image of the contents or components thereof relative to each other in three-dimensional space and to obtain some indication of likely composition. The thicker or more dense an object is then the more it will attenuate an x-ray beam. By use of suitable detectors and a suitable source, radiographs of an item under screening in the form of images based on the absorption of an object or set of objects can be generated.
It is known that spectroscopic information from transmitted x-rays could be used to give additional information about the material content of the objects or components being scanned. It is known that the x-ray absorption properties of any material can vary spectroscopically, and that this effect depends in particular on atomic number. This has led to development of dual-band or dual-energy detectors which are capable of separately identifying low- and high-energy bands from the full spectrum of x-ray emissions. More recently, the development of detectors that can resolve spectroscopic information about the transmitted x-rays more effectively has led to the development of apparatus that discriminate across a larger range of bands and generate a larger plurality of images. For example U.S. Pat. No. 5,943,388 describes a system that makes use of cadmium telluride detectors to image across at least three energy bands and generate at least three images.
Such systems allow the presentation of transmission radiographs with spectroscopic energy resolution, which can assist in the declutter of extraneous information relative to that presented in broad spectrum monochromatic images presenting merely average intensity across the spectrum, especially where an object comprises multiple component elements or materials in the transmission path. They do not always collect data in such a way as to allow derivation of characteristic material information. It is desirable to do so more effectively.
Even with this resolution, such devices can still be confused by objects which are superimposed in the x-ray path. Moreover, they generally will give no information concerning the crystalline or polycrystalline nature of an object.
Polycrystalline materials scatter x-rays and, the resulting x-ray image may hardly detect such polycrystalline material because a very large portion of the x-rays which have not been absorbed by the material will have been scattered and so not received by the detector. This is unfortunate as in security x-ray screening a number of threat items are polycrystalline in nature, in particular plastic explosives such as CP4, RDX, PETN and proprietary formulations thereof, drugs and the like and are therefore difficult to detect by using conventional x-ray systems.
U.S. Pat. No. 5,313,511 outlines a system for producing separate images for the transmitted beam detected using dual energy detectors, the backscattered beam and the forward scattered beam. The forward and back scattered beam images contain only intensity information and cannot be combined with the transmission image for more accurate imaging and materials identification.